1. Field of the Invention
This invention relates generally to plasma processing systems, and more particularly to apparatus and methods for making in-situ measurements of plasma properties in plasma processing systems.
2. Brief Description of the Prior Art
Single-ended Langmuir probes and other diagnostic probe methods have been widely used in industry to characterize plasmas, and more specifically plasmas used in low pressure vacuum processing systems. Measurement techniques have been described in reference books including Swift and Schwar, Electric Probes for Plasma Diagnostics, (American Elsevier, New York, 1969) and by Hershkowitz in Plasma Diagnostic, Vol. 1, Discharge Parameters and Chemistry, ed. Aucciello and Flamm (Academic, New York, 1989), pp. 113-183. The use of single-ended Langmuir probe diagnostic methods for monitoring plasma processing environments is also known, along with methods for operating the probes as generally needed for plasma characterization. For example, in U.S. Pat. No. 4,006,404, Szuszczewicz et al. discuss excitation of a single-ended Langmuir probe through pulsed modulation so as to avoid problems associated with sampling and surface contamination. In U.S. Pat. No. 5,339,039, Carlile et al. describe a single-ended Langmuir probe system that incorporates radio frequency (RF) compensation and tuned-filtering for operation in RF powered plasma environments. In U.S. Pat. No. 5,167,748, Hall describes using one or more single-ended Langmuir probes to measure charged particle density and electron temperature in order to monitor the state of the plasma and thereby control the plasma geometry within a processing system.
More recently, diagnostic probe assemblies on various components of plasma-based processing systems, such as low pressure semiconductor processing chambers, have been described. Exemplary of this work is U.S. Pat. No. 5,451,784 to Loewenhardt et al., wherein plasma probes and ion energy analyzers are include on a composite diagnostic wafer that is then disposed into a plasma processing system in order to characterize the plasma properties adjacent to the workpiece surface. Similarly, Ke et al. in U.S. Pat. No. 5,989,349 describe the use of planar probes embedded in a semiconductor wafer process diagnostic pedestal for the purpose of monitoring ion currents from the plasma and DC bias potentials. Hikosaka, et al. in U.S. Pat. No. 5,471,115, describe a method for measuring plasma properties using a high frequency plasma oscillation probe that measures absolute electron density in the plasma, with the intent to feed such information back to a main control system that can adjust RF power, gas flows or operating pressure. Also, Booth et al. in U.S. Pat. No. 5,936,413 describe using a capacitively isolated, single-ended planar probe that is excited with an RF voltage waveform to obtain plasma characteristics while avoiding deposition and probe surface contamination and disruption of the processing plasma.
All of the techniques described above have limitations that restrict their usefulness in obtaining real-time measurements of plasma conditions within commercial plasma processing systems. Many of these measuring devices are intrusive in that they require the use of a probe that protrudes into the plasma body, which inherently disrupts the plasma properties when processing materials. Moreover, the use of any single-ended probe is intrusive, particularly when the probe is forward biased at or near the plasma potential, resulting in a condition that necessarily disrupts the electrical structure of the plasma body during processing. Many of the techniques described are intended for experimental characterization of non-corrosive or non-depositing plasmas, and are thus not intended to collect plasma measurements under commercial processing conditions. Finally, these teachings typically provide only a single-point approach to monitoring the plasma properties, and as such provide no means of determining the global or spatial properties of the processing plasma at its boundary.